The present invention relates to the use of water-soluble carbohydrates and phenolic compounds as thermosetting adhesives.
Typically, exterior grade composite and panel wood products are bonded with synthetic adhesives. Phenol-formaldehyde resin adhesives are used in more than 90% of exterior grade composite products. However, the finite supply of fossil fuels, coupled with an increasing global demand for wood products, has created a need for an alternative, environmentally sound adhesive system based on renewable resources. At the same time, the need to produce uniform and high quality composite products from lower cost raw material such as smaller diameter logs, wood processing by-products such as sawdust and bark, and agricultural wastes, has created a need for improved bonding products and processes.
Renewable resources, such as carbohydrates and phenolics, and particularly those of a lignocellulosic nature, have been investigated as a possible source of constituents suitable in exterior grade adhesives. To date, few adhesive systems based on renewable source have been successfully adapted for commercial application. Generally, such adhesives do not possess adequate properties, suffering from weak bond strength and poor handling qualities, to meet the standards required of composite products. Those renewable source adhesives capable of meeting today""s composite product standards are usually too expensive to compete with existing synthetic resins, often requiring extensive chemical modification and/or extremely long curing times.
Nonetheless, the components of lignocellulosic materials have been extensively studied in hopes of finding a renewable source adhesive suitable for commercial use in composite wood products. Wood, similar to other plant resources, is composed of about 45 to 50% cellulose, 20 to 35% hemicellulose, 20 to 25% lignin, and greater than 0 to 10% extractives. Chemically, cellulose is the main structural component of all plant cell walls, and is a linear polysaccharide built up from anhydro glucose units having the general formula (C6H10O5)n. Cellulose is the most abundant source of carbohydrates in plants. Upon acid hydrolysis cellulose converts into glucose.
Hemicelluloses are a group of heterogenous polysaccharides that, next to cellulose, constitute the most abundant sources of carbohydrates in plants. Hemicellulose, associated with the cell wall, is highly soluble in alkali and is relatively easy to degrade by an acid hydrolysis into simple sugars or sugars acids. Hemicellulose may be repeated by the general formula (C5H8O4)n1 (C6H10O5)a2, representing five and six carbon sugars such as pentosan and hexosan. Upon acid hydrolysis hemicellulose converts into various reducing sugars.
The third major component of lignocellulose is lignin, which is phenolic in character and acts as the natural binder within the lignocellulose to cement cells together. The precursors of lignin and its formation in lignocellulosic material are still not completely understood, although recently, considerable progress has been made in this area of research. It is generally accepted that lignin is a polymeric material composed of phenyl propane units that are linked together by carbon to carbon as well as carbon to oxygen (ether) bonds.
The minor component of lignocellulose consists of extractives. All plants and agricultural vegetation contain a number of organic substances that can be extracted with organic solvents, or in some cases, with water. Among these belong aliphatic, aromatic and alicyclic compounds, hydrocarbons, alcohols, ketones, and various types of acids and phenol compounds. Furthermore, sterols, tannins, essential oils, resins, dyestuffs, proteins, wax, and some alkaloids are found.
Carbohydrates have been explored in the past as both coreactants with phenolic resin and as the sole ingredient in adhesive. Meigs (U.S. Pat. No. 1,593,342, 1,801,053 and 1,868,216) carried out some of the early work with a phenol-carbohydrate combination. Meigs was searching for a process to produce solid, fusible, thermoset molding compounds. The reactions used both acidic and base catalysts and often introduced coreactions with aniline or aliphatic amines. In other examples, Chang and Kononenko (Sucrose-Modified Phenolic Resin as Plywood Adhesives, Adhesives Age 5(7):36-40, 1962) developed an adhesive system by coreacting sucrose in a phenol-formaldehyde resin formulation under alkaline conditions. More recently, Gibbons and Wondolowski (Can. Pat. 1,090,026) reacted carbohydrates with phenol and urea, or a diamine, in the presence of an acid catalyst to produce a fusible resin for bonding wood products.
Other investigators have used acidic conditions only to produce carbohydrate-based resin. Mudde (Corn Starch: A Low Cost Route to Novolac Resins. Mod Plast. 57(2): 69;74, 1980) described a method that relied on the acidic conversion of starch to 5hydroxymethyl furfuralxe2x80x942 formaldehyde for condensing with phenol in a Novolac resin. Turner et al., (DE-A-1,905,054) investigated carbohydrates, not involving phenol as a coreactant, and produced a water-resistant adhesive. Turner et al. degraded pentose and hexose sugars with acid, while coreacting with such materials as formaldehyde, furfural alcohol, polyvinyl alcohol, or amines to produce a thermosetting adhesive suitable for particle board production. As another example, Stofko (U.S. Pat. Nos. 4,107,379 and 4,183,997) proposed formulations that used a variety of carbohydrates sources and reactions under acidic conditions with different modifiers to produce thermosetting water-resistant adhesives.
K. C. Shen (U.S. Pat. No. 5,017,319, EP-B-0,161,766 and EP-A-0,492,016) converted ligno-cellulosic material directly into both thermosetting resin adhesive and composite products by selectively hydrolyzing and decomposing hemicellulose and/or cellulose fractions, using high pressure steam, into low molecular weight water-soluble resin material including pentose and hexose sugars, sugar polymer, furfural, hydroxymethyl furfural, dehydrated carbohydrates, organic acids and other decomposition products. The water-soluble resin material alone, thus produced, can be used in liquid or powder form as a thermosetting water-proof resin adhesive.
Since lignin is believed to be the natural binder within lignocellulose and is phenolic in nature, it has been extensively studied and researched in the past hundred years as a binder for lignocellulosic composite products.
In U.S. Pat. No. 726,029, A. Classen used steam to treat saw dust with acid and then cooked the acidified saw dust under pressure at a temperature of 105 to 145xc2x0 C. for 30 to 60 minutes to render the hemicellulose water-soluble. After cooking, the treated mass was washed with water to remove the water-solubles before drying and molding into composite products. U.S. Pat. No. 2,303,345 by Mason and Boehm described a process of making strong products from lignocellulosic material. Mason and Boehm employed high pressure steam to separate lignin from lignocellulosic material for bonding. In their process, the hemicellulose was hydrolysed into water-solubles and removed from the treated lignocellulose before the fibres and lignins were made into hard board. Under high temperature and pressure lignins were melted as a binder to cement fibres a into a high density hard board. The water-solubles, consisting mainly of reducing sugars, were concentrated into wood molasses commonly used in animal fodder. Although the well-known Masonite process of making hard board, using natural lignin as binder, has been in commercial production since the 1930s in USA and other countries, it is now an industry in decline due to inefficiencies and environmental concerns about the enormous quantities of waste water.
Other investigators have used lignin from the pulping industry in combination with phenol-formaldehyde resin to form copolymer resin adhesives. In U.S. Pat. No. 2,786,008, R. Herschler incorporated ammonium based spent sulfite liquor, or lignosulfonates, a by-product of sulfite pulping industry, into an acid tolerant thermosetting phenolaldehyde resin for bonding plywood and fibre board. In UK-A-1,404,536, K. Forss proposed a system of incorporating high molecular weight lignins, fractionated from lignosulfonates, into a phenol-formaldehyde resin to yield a copolymer thermosetting adhesive for bonding plywood and fibre board in sulfite pulping, both lignin and hemicellulose fractions are solubilized during acidic cooking and washed off as spent sulfite liquor effluent. In the past, this waste material was discharged into rivers and streams causing severe water pollution. Today, pulp mills in North America are equipped to recover the waste liquor and use it for boiler fuel. A very small fraction of the spent sulfite liquor is processed into useful products for industrial application.
U.S. Pat. No. 4,193,814, issued to K. C. Shen, describes a method of using calcium, sodium, magnesium and ammonium based spent sulfite liquor, or lignosulfonates as a thermosetting adhesive for bonding wood composite products. This process involved treating the spent sulfite liquor with sulfuric acid to activate the lignosulfonic acids and convert the spent sulfite liquor into a highly acidic thermosetting adhesive. Further, in U.S. Pat. No. 4,265,845 K. C. Shen et al. found that ammonium based spent sulfite liquor, without any modification or addition of chemicals, could be thermoset to yield a water-proof bond. Fractionation of the spent sulfite liquor to contain a high proportion of the low molecular weight fraction further improved the adhesive quality. However, the improved spent sulfite liquor adhesive still required high press temperature and long press time to thermoset or cure into a water-proof bond.
Trees contain about 15 to 20% by weight of bark. Therefore the wood processing industry generates enormous amounts of bark as waste which at present are simply being used as boiler fuel having a very low economic value. The barks of various trees and certain agricultural wastes contain organic extracts of which phenolic compounds are the major constituents. These extracted phenolic compounds constitute the starting material for the production of adhesives and are available from the wood industry""s own resources and from agricultural resources. A waste material itself, bark is generally a much richer source than wood for extracts, the most important being monomeric polyphenol or flavonoid compounds, and polymeric phenolics, such as tannins, phlobophenes and phenolic acids. There is no real difference between phenols and tannins since both are phenolic in character. There are two types of tannins: condensed and hydrolyzable. Condensed tannins, using phenolic components from bark extract or from certain agricultural residues, with the addition of formaldehyde, have been used from time to time as a resin adhesives for bonding plywood and particle board. Tannin extractives from the bark of Acacia (wattle of mimosa bark extract) and the wood of Quebracho normally fortified with a small portion of synthetic phenol-formaldehyde resin have been used in the commercial production of exterior grade plywood and particle board in certain countries. However, there are practical and cost limitations associated with the use of tannin adhesives, which have restricted their application for wider commercial use.
It is well known that carbohydrate and lignin based adhesives have not achieved commercial success due to some fundamental flaws, such as high acidity, slow curing rates/longer press times, and low tolerance to high moisture content during processing. Similarly, tannin-formaldehyde adhesives suffer from a variety of short-comings, such as weak cohesive strength, short pot-life, pre-cure and high viscosity. These problems not only affect bonding qualities, but also pose difficulties in production, processing and handling.
It has been found, however, unexpectedly and surprisingly that prehydrolysis of lignocelluloses with acids or acid liberating chemicals under optimum conditions without washing out the hydrolysis products enhance the bondability of lignocellulosic materials with tannin-formaldehyde resins. This was insofar unexpectedly as the conventional teaching is that the addition of carbohydrates is detrimental to the overall bonding quality of tannin-formaldehyde bonded boards (see Pizzi, A. xe2x80x9cTannin Based Wood Adhesivesxe2x80x9d at page 215, Wood Adhesives: Chemistry and Technology, Marcel Dekker, New Yok: 1983, and Hemingway et al. xe2x80x9cCondensend Tannin: Problems and Prospective for their extended Use in Wood Adhesivesxe2x80x9d at page 164, Wood Adhesives in 1985: Status and Needs: Proceeding of the Conference with the Forest Products Research Society).
The prehydrolysis step leading to the formation of degration products of carbohydrates and lignin seems to have a dual effect on bonding. It increases the physical accessibility of the lignocelluloses towards bonding. Moreover, the degration products seem to have inherent bonding potential as they copolymerise with tannin-formaldehyde resin used for bonding.
It has now been found also, however, that copolymer adhesives of carbohydrates, lignin and tannins alleviate common problems inherent to carbohydrates, lignin and tannin-formaldehyde based resins, such as high acidity, slow curing, low cohesive strength, short pot life, high viscosity and pre-curing. The incorporation of black liquor or wood molasses and spent sulfite liquor or lignosulfonate, waste by-products from the hard board and pulping industries, respectively, into the copolymer resin system makes it economically efficient and environmentally attractive.
Renewable source thermosetting adhesives entirely based on lignocellulose vegetation offer permanent solutions to the wood industry. Although tannins are relatively expensive and limited in production quantity, up to 400,000 tons per year worldwide for condensed tannins, a thermosetting adhesive consisting of a minor portion of tannin and a major portion of carbohydrates and lignins would provide a significant economic advantage to the wood industry. The lower cost adhesives will allow for the application of higher resin content to produce new and/or better quality composite products.
Insofar the present invention provides: an adhesive composition comprising a product produced by copolymerization of one or more phenolic compounds and one or more water-soluble carbohydrates; or a mixture comprising one or more phenolic compounds or one or more water-soluble carbohydrates, the one or more phenolic compounds being copolymerizable with the one or more water-soluble carbohydrates.
It may be preferable that the carbohydrates comprise reducing sugars or other reducing carbohydrates.